Dear mailing list, This question relates to an EXAFS analysis approach I am currently investigating in respect to a number of iron-sulfur clusters with nuclearities of 2, 3, 4, 5 and 6. The analysis is easy enough to perform when the Fe atoms in the clusters are chemically equivalent (i.e. same oxidation states and coordination environments) and can be simplified to a single Fe absorber model. The analysis gets a little tricky when the oxidation states of the Fe atoms, and also coordination environments are different – hence the sites become chemically inequivalent. The approach that I use takes into account these different environments and resulting populations for each of the iron atoms, again collapsing into a single Fe absorber model and I am able to obtain nice fits of the data with bond distances in close agreement with compounds in which these distances have been obtained through crystallography. The DW’s, visual fit and statistics are also well behaved and I have taken into consideration the number of refined parameters to ensure a well over-determined fit to the models (i.e. the data is not simply fitting to anything with lots of fitting variables). The main hesitation I have, and reason for me posting this question, is for the compounds in which the Fe atoms are in different oxidation states we would expect there to be differences in the E0 of these atoms. I know that for every increase, by one, in oxidation state (i.e. Fe(I) to Fe(II)) there is a shift in E0 to higher energy by roughly 2 to 3 eV (as can be observed in the XANES from studies conducted on mononuclear Fe compounds). I am really not sure as to what extent this effect may alter the EXAFS signal for a cluster. From what I observe in the fitting I would say not much, but I would like to know if this has been encountered, or thought about before by anyone else (i.e. maybe in reference to EXAFS studies on other homonuclear metal cluster systems in catalysis). Regards, Mark -- Mark Bondin PhD student School of Chemistry, University of Melbourne (03) 8344 7015 (Tower lab) 0409 252 983 (mobile)